1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (to be referred to as an MRI apparatus hereinafter) and, more particularly, an improvement in a radio frequency coil (to be referred to as an RF coil hereinafter) for receiving a magnetic resonance signal from an object to be examined.
2. Description of the Related Art
As an example of such an RF coil, a surface quadrature (QD) coil or a volume QD coil is known. Examples of the volume QD coil are a saddle coil, a slotted tube resonator coil, and the like.
The surface QD coil is suitable for imaging a surface region of the object. However, the surface QD coil does not have a sufficient sensitivity to cover a deep portion of the object, so that it is difficult to image a deep portion of the object with a high S/N ratio.
The volume QD coil has a saddle or cylindrical shape, and an object is placed in the coil. Thus, the volume QD coil has a large size. Even when a small local portion in the object is to be examined, since an unnecessary portion is included in the sensitivity region, unnecessary noise is detected. Hence, the volume QD coil is not suited for obtaining an image of a small local portion with a high S/N ratio.
A non-QD coil-pair assembly in which rectangular or circular coils oppose each other is also known. FIG. 1 shows an RF coil-pair assembly in which a pair of rectangular coils 2 and 4 are arranged to oppose each other to sandwich an object (not shown) in order to detect a magnetic resonance (MR) signal from the object. Reference symbols T denote circuits usually called trapping circuits. The trapping circuits T serve to protect circuits connected to the RF coil-pair assembly and to eliminate disturbance in the excited magnetic field caused by an induced current. More specifically, an RF pulse is generated by a transmission coil (not shown) in order to excite the object. An induced electromotive force is generated in the RF coil-pair assembly by the RF pulse. The trapping circuits T prevent a current caused by the induced electromotive force from flowing in the RF coil-pair assembly. Reference symbol Ct denotes a variable tuning capacitor for tuning the resonant frequency of the RF coil-pair assembly to the Larmor frequency of the MRI apparatus. The rectangular coils 2 and 4 are connected to each other to provide an output signal. The output of the RE coil-pair assembly is supplied to a data processor (computer system) for reconstructing an image through a pre-amplifier 6 and a receiver/DAS (data acquisition system) 8.
FIG. 2 shows the magnetic distribution (sensitivity distribution) generated by the RF coil-pair assembly. Generally, in an MRI system using a superconducting magnet as a static magnetic field generating unit, the direction of the static magnetic field is the Z direction (the direction of the body axis of the object) and the MRI system is called a horizontal static magnetic field type. Since the RF magnetic field generated by the RF coil-pair assembly of FIG. 1 is Y direction, as shown in FIG. 2, and substantially perpendicular to the static magnetic field (Z direction), it is suitable for examination of the abdomen or the like by an MRI system using a superconducting magnet. In this coil-pair assembly, since the side end portion and its vicinity of the object are not in the sensitivity region, if the region of interest (ROI) is located in the central portion of the coil-pair assembly, unnecessary noise produced by an unnecessary portion is not detected, and the size of the coil assembly is small when compared to that of a volume QD coil.
In an MRI system using a permanent magnet as a static magnetic field generating unit, however, since the direction of the static magnetic field is generally the Y direction (vertical magnetic field), the RF coil-pair assembly having the arrangement shown in FIG. 1, i.e., the RF coil-pair assembly having an RF magnetic field component in the Y direction, as shown in FIG. 2, cannot detect an MR signal.
Furthermore, since this coil-pair assembly is not a QD coil, it cannot perform imaging with a sufficiently high S/N ratio.
In this manner, each of the conventional RF coils, e.g., the surface QD coil, the volume QD coil, the coil-pair assembly, and the like has both advantages and disadvantages, and it is difficult for any of the conventional RF coils to image an ROI with a high S/N ratio.